Scanning electron microscope

ABSTRACT

BY THE SAID MAGNETIC FIELD AND REMOVED FROM THE MAGNETIC FIELD ALONG THE OPTICAL AXIS.   A SCANNING ELECTRON MICROSCOPE EQUIPPED WITH A HIGH EXCITATION ELECTRON LENS IN WHICH THE SPECIMEN TO BE ANALYZED IS PLACED IN A MAGNETIC FIELD PRODUCED BY THE LENS. IN THIS ARRANGEMENT, THE INCIDENT ELECTRON BEAM IS FOCUSED AND DEFLECTED BY THE MAGNETIC FIELD BEFORE THE SPECIMEN. FURTHER, SECONDARY ELECTRONS EMITTED FROM THE SPECIMEN BY THE IRRADIATION OF THE ELECTRON BEAM ARE FOCUSED SPIRALLY

Feb. 20, 1973 HIROTAMI KOIKE ETAL 3,717,761

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SCANNING ELECTRON MICROSCOPE Filed Apri1'14, 1971 4 Sheets-Sheet 4 United States Patent Office 3,717,761 Patented Feb. 20, 1973 3,717,761 SCANNING ELECTRON MICROSCOPE Hirotami Koike and Katuyoshi Ueno, Tokyo, Japan, as-

signors to Nihon Denshi Kabushiki Kaisha, Tokyo,

Japan Filed Apr. 14, 1971, Ser. No. 133,894 Claims priority, application Japan, Apr. 18, 1970, 45/ 33,284 Int. Cl. Htllj 37/26 U.S. Cl. 250-495 PE 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a scanning electron microscope and particularly to a scanning electron microscope in which secondary electrons emitted from a specimen are effectually detected.

In the scanning electron microscope secondary electrons are emitted from a specimen surface by irradiating the specimen with an incident electron beam caused to scan the specimen by a deflection coil. The secondary electrons are detected by a detector located between the final condenser lens and the specimen. However, since the secondary electrons disperse in many directions, effective detection is diflicult or impossible.

In recent years, attempts have been made to observe a secondary electron image using a transmission type electron microscope. With this type of microscope the specimen is inserted in a limited inner part of the pole piece in the objective lens. Unfortunately, there is insufficient room to house the secondary electron detector in such a limited space. As a result, an electron microscope capable of observing both a transmission image and a secondary image has thus far not been realized.

It is an advantage of scanning electron microscopes according to this invention that they effectually detect the secondary electrons emitted from the specimen in the usually dispersed manner. It is a further advantage that the specimen may be observed over a wide field of vision. Also, a high resolution secondary electron image is observed. It is a further advantage of electron microscopes according to this invention that they are capable of providing both a secondary electron image and a conventional transmission electron image.

Briefly, according to this invention, the specimen is situated within a high excitation objective lens. The prefield (that part of the magnetic lens field before the specimen) focuses the incident beam and confines the secondary electrons emitted from the specimen. The post-field (that part of the magnetic lens after the specimen and usually below the specimen) focuses and magnifies the beam which passes through the specimen. By so doing, both a scanning secondary electron image and a normal transmission electron microscope image can be observed by the one electron microscope.

The aforedescribed drawbacks have been overcome in this invention by positioning the specimen in a magnetic field produced by a high excitation electron lens. The incident electron beam is focused by the pre-field prior to irradiating the specimen. Consequently, secondary electrons are emitted from the specimen. Since the energy of the said electrons is low, the electrons are spirally focused along the optical axis by the pre-field and removed therefrom. By so doing, the secondary electrons are effectually detected.

Various other objects and advantages of this invention will become readily apparent from the following detailed description with reference to the accompanying drawings in which:

FIG. 1 shows a scanning electron microscope in which a specimen is positioned in a magnetic field produced by an electron lens;

FIG. 2 is an analogous optical view of a modified embodiment of the microscope shown in FIG. 1;

FIG. 3 shows an electron microscope in which a specimen is positioned in an objective lens;

FIG. 4 is an analogous optical view of the electron microscope shown in FIG. 3;

FIGS. 5, 6 and 7 show modified embodiments of the electron microscope shown in FIG. 3;

:FIG. 8 shows the magnetic distribution of the objective lens used in FIG. 7; and,

FIGS. 9 and 10 show other modified embodiments according to this invention.

Referring to FIG. 1, an electron beam produced by an electron gun 1 passes through a condenser lens 2. A coil 3, in the condenser lens 2, is supplied with excitation current by a power source 4, thereby enabling the electron beam to be focused by the magnetic field produced by the lens 2. The focused electron beam is then deflected by, for example, a magnetic field produced by a deflecting means 5 and irradiated on a specimen 6 which is placed within the magnetic field produced by a high excitation electron lens 7. A scanning signal is supplied to the deflecting coils 5 by a scanning signal generating circuit 13 causing the incident electron beam to scan the surface of the specimen 6.

Secondary electrons are emitted from the specimen by the irradiation of the incident electron beam. Furthermore, since the pre-field of the electron lens 7 extends to the upper part of the deflection means 5, the secondary electrons are spirally focused along the electron optical axis in the upper direction. Finally, the said electrons are detected by a detector 8 and a signal is supplied to a display means 12. To ensure that the secondary electrons are effectually detected, it is necessary to apply a positive voltage to the detector 8.

At the same time, the electrons which pass through the specimen are detected by a detector 9 such as a Faraday cage. The output signal from either of the two detectors 8 and 9 are selected by a switch 10', the selected signal being applied to the display means 12 via an amplifier 11. Further, the scanning signal generated by the scanning signal generating circuit 13 is fed into a deflecting means 14 forming part of the display means 12. As a result, when the signal detected by the detector 8 is fed into the said display means, the scanning secondary electron image is observed. On the other hand, when the signal detected by the detector 9 is fed into the display means 12, the scanning transmission electron image is observed.

In this arrangement, since the specimen 6 is positioned in the electron lens 7, the incident electron beam is not affected by extraneous external electrical and magnetic disturbances. Further, since a high excitation magnetic lens is used, both chromatic and spherical aberrations are reduced considerably. Consequently, the irradiation spot of the incident electron beam is very fine and a high resolution scanning electron image is observed.

In another embodiment employing the scanning electron microscope as shown in FIG. 1, a variable power source 16 is used for feeding an excitation current into the coil 15 of the electron lens 7. FIG. 2 shows ananalogous optical view of this embodiment. In the figure, the incident electron beam BB is focused by the condenser lens 2 and deflected by the deflecting means 5. The deflected electron beam is focused and deflected by the electron lens 7 and irradiated on the specimen 6. Now, if the current fed from the power source 16 into the coil 15 is small, the incident electron beam is deflected as shown by BB That is to say, the incident electron beam scans the specimen ,over a wide field of vision. As the excitation current is increased, the scanning range is reduced as shown by EB;;, and BB FIG. 3 shows an electron microscope capable of observing both the normal transmission electron microscope image and the scanning electron image. In this figure, the objective lens 30 comprises a pole piece 31, a yoke 32 and a coil (not shown). The pole piece 31 includes three ferromagnetic poles 33, 34 and 35 connected by a non-magnetic spacer 36, poles 33 and 34 constituting a gap, to be referred to as the first gap, and poles 34 and 35 constituting another gap to be referred to as the second gap. In this way, a strong magnetic field is produced which acts as a lens in each of the two gaps when high excitation current is applied to the coil. A specimen 37 is placed between the two gaps preferably in the object plane of the objective lens. The field in the first gap focuses the parallel electron beam and the beam forming a reduction image of a crossover on the specimen 37. An advantage of this arrangement is that, since the magnetic field in the first gap is strong, the reduction image and its accompanying spherical aberration are both very small. A further advantage of this arrangement is that the specimen 37 is unaffected by external electrical and magnetic disturbances.

Secondary electrons dispersed from the specimen 37 are confined by the pre-field and accelerated in the upper direction by an electrode 39 to which a positive voltage is fed from a power source 38. By so doing, the secondary electrons are spirally eccelerated in the prefield along the electron beam axis since the energy of the secondary electrons is weak and the said pre-field is strong.

The electrons which pass through the opening in the electrode 39 are deflected by optional magnetic field 40 (it is possible to eliminate this field as required) and directed towards a detector 41 located away from the electron beam axis. The detector 41 comprises a grounded case 42, a photomultiplier 43, a light tube 44, an electrode 45 to which a positive voltage is fed from the power source 38 and a fluorescent screen 46. The secondary electrons impinge on the fluorescent screen 46 and are changed into light signals. These signals then pass through the light tube 44 and are detected by the photomultiplier 43. The output of the detector 41 is fed into a display means 47 such as a cathode ray tube.

A pair of deflection coils 48, to which a scanning signal is fed by a scanning signal generating means 49, are arranged in the upper part of the magnetic field 40 so that the incident electron beam scans the specimen 37 in two directions. The scanning signal is also fed into a deflecting means 47a of the display means 47. Referring to FIG. 4, the incident electron beam EB is scanned as BB BB and EB,,-. The magnetic field produced by the first gap represented by the first lens 0L of the objective lens 30 focuses and deflects the incident electron beam.

In the above described embodiment, the detector 41 is positioned in a higher plane than the specimen so as to detect the secondary electrons. It is equally possible of course to position the said detector in a lower plane than the specimen so as to observe the scanning transmission electron image. When observing a transmission electron microscope image, however, the deflecting means 48 should be deactivated and the condenser lens (not shown) adjusted so that the incident electron beam focuses on the front focal plane of the first lens 0L This is necessary in orderto irradiate a parallel electron beam on the specimen 37. The electron beam which passes through the specimen is focused on the plane P by the magnetic field produced by the second gap represented in FIG. 4 by the second lens 0L The image, thus obtained, is magnified by a magnifying lens or lenses and projected onto a fluorescent screen.

FIG. 5 shows a modified embodiment of the electron microscope shown in FIG. 3. In this embodiment, a second electrode 50 is provided in place of the magnetic field 40 shown in FIG. 3. The said electrode 50 is maintained at ground or negative potential so that the velocity of the secondary electrons accelerated by the first electrode 39 is reduced. The said electrons are directed to the detector 41 by the electric field produced by the electrode 45.

FIG. 6 also shows a modified embodiment of the electron microscope shown in FIG. 3. In this case, the objective lens 51 comprises two magnetic poles 52 and 53, a yoke 54 and a coil 55. A permanent magnetic 56 is provided at the upper part of the lens. A yoke plate 57, to which a magnetic pole 58 is fitted, is secured to the upper end of the permanent magnet 56. Consequently, the magnetic field produced by the magnetic poles 58 and 52 acts as the first lens and the magnetic field produced by the magnetic poles 52 and 53 acts as the second lens. It is possible to provide a magnetic excitation coil instead of the permanent magnet 56.

FIG. 7 shows another modified embodiment of the electron microscope shown in FIG. 3. In this case, a pole piece 61 in an objective lens 60 comprises two magnetic poles 62 and 63. The lens intensity to of the objective lens is expressed as follows:

where e is the electric charge, B the maximum valve of the axial magnetic field, d, the half-value width of the axial magnetic field, distribution, m the mass of the electron and V, the accelerating voltage, In this embodiment, to is 2 or over.

FIG. 8 shows the magnetic distribution produced by the objective lens 60 under conditions of strong excitation. The specimen 37 is placed near the maximum intensity area. A magnetic field H produced in front of the specimen 37 acts as the first lens and a magnetic field H produced at the rear of the said specimen acts as the second lens. As a result, the scanning electron image and the transmission electron microscope image are observed in the same way as in the embodiment shown in FIG. 3. When the transmission electron microscope image is observed, it is desirable to adjust the excitation current fed into the objective lens so as to defocus the incident electron beam onto the specimen and thereby irradiate the said beam over a wide field of vision.

In the embodiment shown in FIG. 9, the detector 41 is located above the deflecting means. However, since the premagnetic field extends to the upper part of the deflecting means 48, the electrons are refocused along the optical axis by the said field and detected by the detector 41, even if the secondary electrons are deflected. The advantage of this arrangement is that the deflecting means can be placed nearer the objective lens, thereby minimizing electron beam aberration produced by the deflection, and, by so doing, enabling the realization of a high resolution scanning electron microscope. Further, it is possible to spread the field of vision of the specimen and to locate attachments such as specimen heating and cooling devices with ease.

FIG. 10 shows another embodiment of this invention. Here, the deflecting means 48 for deflecting the incident electron beam is attached to the upper portion of the magnetic pole 62 of the objective lens 60, A movable stage 65 supporting a specimen holder 66 is mounted on the objective lens via ball bearings 64. The detector 41 located in the specimen chamber 67 detects the secondary electrons emanating from the specimen holder 66. Further, the condenser lens 68 is mounted on the upper side of the specimen chamber 67.

Having thus described the invention with the detail and particularity as required by the Patent 'Laws, what is desired protected by Letter Patent is set forth in the following claims.

We claim:

1. An electron microscope comprising:

(A) an electron gun;

(B) a means for deflecting the electron beam produced by the electron gun to scan a specimen;

(C) a high excitation objective magnetic lens in which the specimen is placed, the magnetic field before the specimen confining the secondary electrons emitted from the specimen, the magnetic field after the specimen focusing the electron beam transmitted through the specimen;

(D) a detecting means for detecting the secondary electrons emanating from the specimen; and,

(E) a display means in circuit with the said detecting means for displaying the scanning electron image, the said display means being synchronized with the said deflecting means.

2. An electron microscope according to claim 1 wherein the objective lens comprises three magnetic poles, a yoke and an excitation coil, the first and second magnetic poles producing a magnetic field before the specimen, the second and third magnetic poles producing a magnetic field after the specimen.

3. An electron microscope according to claim 1 wherein the objective lens comprises two magnetic poles, a yoke and an excitation coil, the specimen being placed in the area of maximum magnetic field intensity produced by the two magnetic poles.

4. An electron microscope according to claim 1 wherein the detecting means is located in the upper part of the deflecting means.

5. An electron microscope comprising:

(A) an electron gun;

(B) a deflecting means for deflecting the electron beam produced by the electron gun to scan a specimen;

(C) a high excitation objective lens in which the specimen is located, the magnetic field before the specimen focusing the secondary electrons emitted from the specimen, the post-magnetic field after the specimen focusing the electron beam transmitted through the specimen;

(D) a detecting means for detecting the secondary electrons emanating from the specimen;

(E) a means for directing the secondary electrons from the optical axis to the said detecting means; and,

(F) a display means in circuit with the said detecting means for displaying the scanning electron image, the said display means being synchronized with the said deflecting means.

6. An electron microscope according to claim 5 wherein the means for directing the secondary electrons comprises a magnetic field produced in the optical axis.

7. An electron microscope according to claim 5 wherein the means for directing the secondary electrons comprises an electrode located above the objective lens.

8. An electron microscope comprising:

(A) an electron gun;

(B) a deflecting means for deflecting the electron beam produced by the electron gun;

(C) three magnetic poles provdies along the optical axis, the first and second magnetic poles forming a first gap and the second and third magnetic poles forming a second gap, there being a space between the two gaps where the specimen may be located;

(D) a means for producing a magnetic field in each of the said two gaps, the said fields focusing the secondary electrons and the electron beam transmitted through the specimen respectively;

(B) at detecting means provided above the first magnetic pole for detecting the secondary electrons; and,

(F) a display means in circuit with the said detecting means for displaying the scanning electron image, the said display means being synchronized with the said deflecting means.

References Cited UNITED STATES PATENTS 3,155,827 8/1971 Meyer 250-495 PE 3,155,827 11/1964 Nixon 250--49.5 A

JAMES W. LAWRENCE, Primary Examiner C. E. CHURCH, Assistant Examiner US. Cl. X.R. 250-495 D 

